https://www.environmentalleader.com/2015/12/10/how-to-reduce-carbon-captures-energy-penalty/
[links in on-line article]
[From the article:
"Henderson says the single most important factor that is holding back
deployment of CO2 capture is the high operating cost caused by its
negative effect on plant output and efficiency. Incorporating improved
heat integration will be valuable in reducing this."
Actually, there are likely 2 more important factors.
1) The world is coming to realize we have to leave fossil carbon in the
ground, so building new coal-fired generation is about to become as rare
as building new nuclear fission generation. Retrofitting CCS to
existing obsolete infrastructure is even more expensive and less
effective than implementing it in new build. In short, the EROEI to
capture carbon numbers don't work.
2) It's now less expensive on a life-cycle basis to build
wind-generation plus integrated electrical storage (for daily cycle) so
the renewable is no longer intermittent, but fully dispatchable - not
even counting the environmental and health benefits or carbon credits.
If you have a good wind resource within a couple of hundred kilometres,
it's now hard to justify fossil-carbon generation just on the straight
cost per kWh delivered over the life cycle of the built plant.]
December 10, 2015
How to Reduce Carbon Capture’s Energy PenaltyBy: Jessica Lyons
Carbon capture from coal-fired power plants takes energy. So more coal
has to be used to get the same amount of power from the plant. Thus
reducing the energy demand of carbon capture is vital to making it more
attractive.
In a report for the International Energy Agency Clean Coal Centre, Power
plant CO2 capture heat integration, Dr. Colin Henderson explores various
ways to use the waste heat from carbon capture systems in the power
plant and so reduce the energy penalty of carbon capture. Addition of
CO2 capture systems can result in up to 30 percent loss of electrical
efficiency if there are no integration measures installed, IEA says.
In post-combustion capture, CO2 is scrubbed from the flue gases after
they emerge from conventional gas cleaning systems. An amine solution,
typically monoethanolamine (MEA) at about 40 degrees Celsius in an
absorption column is contacted with the cooled flue gas from the boiler.
The CO2-rich solvent is then heated in a separate desorber vessel to
release the CO2 and regenerate the solvent for reuse.
Large quantities of steam have to be taken from the main plant to
provide heat for this duty. This steam extraction and consequent loss of
power from the turbine typically accounts for about two thirds of the
overall energy penalty of carbon capture. Power to drive fans,
compressors and pumps in the capture systems further reduces net output.
If some of the potential sources of heat from the capture plant are used
in the water-steam cycle this will reduce the amount of steam extraction
needed and the associated drop in gross power. There are large
quantities of waste heat available but their temperatures are not high.
The low-grade nature of the heat is the greatest challenge to increasing
the effectiveness of its use.
Henderson’s report describes the various different approaches such as
adding pressure control valves so that steam arrives at the stripper
reboiler at the correct temperature (usually 120 degrees Celsius) and to
protect the turbines, particularly at times of variable load. Excess
energy in the extracted steam would also be exploited, for example, by
using a heat exchanger for LP feedwater heating or a let-down turbine.
Before transport as a supercritical fluid to storage, captured CO2 is
compressed to around 11 MPa. This compression is carried out in stages,
with cooling in-between to control the working temperature and minimise
the energy needed. The heat from this could be suitable for LP feedwater
heating. If some of the compressor intercoolers are omitted this will
raise the temperature of the recovered waste heat, but means higher
power consumption by the compressor.
Another possibility is to use compression before liquefaction followed
by pumping. This would reduce the need for electrical energy, but it
would be offset by the power decrease in the steam turbine as a result
of steam extraction to drive the refrigeration cycle. An unconventional
CO2 compressor (Ramgen) offers a higher pressure ratio and temperature
rise per stage, so higher grade heat could be extracted. The Ramgen
compressor could result in an increase in overall net power compared to
conventional compression. However, the technology has not yet been
commercialized.
Local ambient conditions may affect the opportunities for improvements
in efficiency. Heat pumps could aid waste heat utilization. Other
methods of optimization have included integration with combined heat and
power systems, addition of low temperature bottoming cycles and addition
of thermo-electrics.
Henderson says the single most important factor that is holding back
deployment of CO2 capture is the high operating cost caused by its
negative effect on plant output and efficiency. Incorporating improved
heat integration will be valuable in reducing this.
Another carbon capture report, released this week at COP21 by members of
the ENGO Network on CCS, found strong carbon capture policies are the
“missing ingredient” in faster adoption of carbon capture and storage
technology to enable CO2 reductions.
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